STM32F042 SWD IDCODE error

[atx]

Posted on August 22, 2015 at 21:51

I have a problem with debugging an STM32F042 board. When the debugger (ST Nucleo) asks for IDCODE, the MCU responds with an error (trace shown below). At first, I thought that the bootloader is the problem, so I flashed a bunch of zeroes onto the start of the flash to disable it, but it persists. Any clues on what might be causing this?

[atx]

Posted on August 24, 2015 at 18:49

Ok, I am an idiot. The problem was, that I had NRST tied directly to Vcc - without any resistor. This caused the MCU to pull the 3.3V power rail down to ground each time the debugger tried to connect.

[Tesla DeLorean]

Posted on August 24, 2015 at 22:31

You aren't the first, and won't be the last, to do that, at least you caught it quickly